In CANs, high levels of reliability are required for the quality of communication, and it is so-called electromagnetic noise in particular that has an adverse effect on reliability. In resolving the problems of electromagnetic noise, it is necessary not only to suppress the generation of the noise so that it will not adversely affect the wireless communication, etc. used inside the automobile, but also to ensure high noise-resistance properties in order to maintain the communication quality even in the face of the noise that seeps in from the surroundings. For instance, in terms of radiated noise, harmonic noise with a signal frequency of 250 kHz (or 500 kHz) is radiated at high levels up to frequency bands of several dozen megahertz.
For its part, Murata Manufacturing Co., Ltd. has commercialized the DLW43SH series of common mode choke coils that are used to suppress the noise in CANs.
【Noise suppression results achieved by DLW series and evaluation of effects on waveforms】
In order to verify the effects achieved by the DLW43SH series in CANs, Murata has evaluated the radiated noise using a CAN evaluation board and also carried out BCI tests and waveform evaluations.
■Radiated noise evaluation
It was verified that a noise suppression effect was achieved by adding the DLW43SH series.
For 250 kHz pulses, the noise suppression effect is approx. 25 dB at a 20.5 MHz vertical polarized wave frequency.
For 500 kHz pulses, it is approx. 18 dB at a 21.0 MHz vertical polarized wave frequency.
With the DLW43SH series placed in the circuit, malfunctioning was suppressed, and 100 mA was cleared at all frequencies (1 MHz to 400 MHz).
With the DLW43SH series placed in the circuit, hardly any effect on the waveforms was found compared with cases where short-circuiting was present.
The star topology shown below was used to evaluate the radiated noise. On the CAN evaluation board, 250 and 500 kHz pulses were input from the clock oscillator to the Txd terminal of the transceiver made by company A to generate the CAN signals.
Open terminations were used for the nodes at the reception to simulate high-impedance ICs.
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